The present invention relates to a device for modulated braking of a weft yarn for textile machines.
More specifically, the invention relates to a device of the type disclosed in the prior EPA No. 99103942.1 by same Applicant, which comprises a set of three fixed yarn guiding eyes, which determine a straight path of the weft yarn through the device, and at least one controlled braking element, which is adapted to engage the yarn in the spaces between said yarn guiding eyes in order to divert its straight path for braking.
In such conventional device, a movable electromagnetic yoke, subjected to the interacting actions of a constant magnetic field and of a current which circulates in an excitation coil, actuates a first oscillating lever with which the braking element proper is rigidly coupled. Said braking element is constituted by a second U-shaped oscillating lever which arranges itself so as to straddle the central yarn guiding eye and engages the yarn in the free portions that lie between the central eye and the end eyes.
The movable yoke is formed by a hollow cylindrical support on which the excitation coil is wound; the coil is immersed in the field of a permanent magnet which generates a flux which circulates in a cylindrical shroud of a material having high magnetic permeance.
Although this conventional device has a response which is generally adequate in terms of reaction to the extremely rapid variations in the mechanical tension of the yarn that occur in modern weaving processes, it is not entirely satisfactory from the constructive point of view, because of a certain mechanical fragility of the movable yoke, whose mass is intentionally very modest, and from the functional point of view, due to some uncertainty in the operation of the movable yoke, which sometimes, especially in the processing of higher-count yarns, can be subject to an electromotive force which is insufficient to produce an adequate response to the device in the above specified terms.